I think some of this is genuine cause for concern, and some of it is a function of the target audience for this kind of offering. We have a lot of visitors who have our site set as their homepage, visit multiple times per day, and/or are heavily engaged in the forum. These people get a disproportionate amount of utility out of some of the enhancements. Removing ads significantly speeds up at least a few parts of our site (depending on what's going down with the ad servers).
There is also a non-significant segment of our traffic that wants long form articles in PDF form. For these people, the subscription may be worth it, and it can easily turn them into repeat visitors (because, hey, why not get the other PDFs too?) who then get more utility out of the other features.
The important bit here is that no one expect this to replace ad revenue anytime soon. It's a diversification that also gives us a nice cross section of visitors to listen to attentively.
There is also a non-significant segment of our traffic that wants long form articles in PDF form. For these people, the subscription may be worth it, and it can easily turn them into repeat visitors (because, hey, why not get the other PDFs too?) who then get more utility out of the other features.
The important bit here is that no one expect this to replace ad revenue anytime soon. It's a diversification that also gives us a nice cross section of visitors to listen to attentively.